The present invention relates to methods and devices for detecting the presence of contaminants in a liquid stream, and is particularly concerned with preventing gas bubbles entrained in the stream from falsifying the detection result.
A number of techniques are known for monitoring liquids for various purposes. For example, in machinery equipped with a system for circulating oil which serves to lubricate bearings, it is advantageous to monitor the oil in order to detect and quantify metal particles which are present in the oil as a result of bearing wear. According to one known technique, the oil is caused to flow through a passage where the oil stream is traversed by a light beam which would be scattered by small metal particles in the oil stream. Any scattered light is detected and the detection result is processed to provide an indication of the presence and concentration of such small particles.
In monitoring systems of this type, it has been found that small air bubbles may also become entrained in the oil stream and can effect scattering of the light beam in a manner analogous to metal particles. This can produce a false indication of the presence of metal particles in the stream, possibly resulting in the generation of an alarm which will lead operating personnel to take inappropriate corrective actions.
It is also known to induce pressure pulsations in a liquid stream by directing a modulated light beam into the liquid so that impurities therein will produce heat that results in periodic thermal expansion of the liquid. Such an arrangement is disclosed in U.S. Pat. No. 4,738,536. An arrangement of this type will produce only a low level of pressure modulation and appears to require the presence of impurities in a sufficient concentration.